US 2006/0141793 A1 discloses a capacitor formed in a trench within a silicon-on-insulator layer. The capacitor is formed between two polysilicon layers separated by an insulating layer.
US 2008/0064178 A1 discloses a capacitor formed in a deep trench in a silicon substrate. The capacitor in made through a silicon-on-insulator layer on top of the silicon substrate. The capacitor is formed between a doped silicon substrate and a second electrode that is made in the trench, separated from the substrate by a dielectric layer.
The above techniques are basically integrable but their capability to store energy leaves room for improvement. Therefore, the attempts to use them as energy storage device may still lead to structures that are impractically large to be integrated on a chip. Known solutions to reach greater specific energies are provided by batteries, fuel cells and supercapacitors, i.e. electrochemical capacitors. Such solutions are described in EP 2104164 A1 and US 2010/0221606 A1 and US 2011/0051322 A1. However, these structures are difficult to integrate firstly because manufacturing processes are not compliant to microelectronics manufacturing processes and secondly because the structures themselves are difficult to accommodate on a semiconductor chip. Thus, the batteries and electrochemical capacitors are practically applicable only as discrete components and therefore require arrangements for contacts with the on-chip applications.
WO 2011/123135 A1 discloses a charge storage device includes first and second electrically conductive structures separated from each other by a separator. At least one of the first and second electrically conductive structures includes a porous structure containing multiple channels. Each one of the channels has an opening to a surface of the porous structure. In another embodiment the charge storage device includes multiple nanostructures and an electrolyte in physical contact with at least some of the nanostructures. Thus, there remains need to seek further improvements in energy storage devices to make them more suitable for use with on-chip applications and energy harvesters.